Currently used long haul optical fiber communications systems typically require signal regenerators. Such devices detect the optical signal, produce a corresponding electronic signal which is amplified, reshaped and, typically, retimed, and then used to drive an appropriate radiation source, thereby producing a fresh optical pulse that is injected into the fiber. However, it has been known for some time that it is possible to amplify, and, under appropriate conditions even reshape, optical pulses without use of electronic regenerators of the type referred to above. In particular, it has been recognized that the Raman effect may be used to amplify optical signals. See, for instance, R. H. Stolen, Proceedings of the IEEE, Vol. 68, No. 10 (1980), pp. 1232-1236, incorporated herein by reference.
Although Raman amplification is possible in fibers other than silica-based (i.e., containing at least 50% by weight, typically&gt;80% by weight, SiO.sub.2) optical fibers, for the sake of concreteness, the exposition below will frequently refer to, and use material constants appropriate for, silica-based fiber. Such fibers have two loss minima in the approximate range 1.2-1.6 .mu.m, and therefore communications systems that use silica-based fiber frequently use signal radiation whose wavelength lies in that range.
Stimulated Raman Scattering (SRS) is known to produce substantial gain in fused silica for frequency shifts in the range from about 100 to about 600 cm.sup.-1, with the maximum gain occurring for a frequency shift of about 450 cm.sup.-1. This means that, in silica-based optical fiber, radiation of wavelengths .lambda..sub.O (to be termed the signal radiation) can be amplified by means of pump radiation that is down-shifted in wavelength from .lambda..sub.O by amounts corresponding to shifts in wave number by about 100 to 600 cm.sup.-1. For instance, for signal radiation of 1.56 .mu.m, the appropriate pump radiation would have a wavelength between about 1.43 and 1.54 .mu.m, with peak amplification taking place for pump radiation of about 1.46 .mu.m. It is also known that there is no inherent threshold power for amplification by SRS, although, in order for usable amplification to take place, a substantial amount of pump power, typically more than 10 mW has to be injected into the fiber, due to the relative smallness of the Raman gain coefficient, which is of the order of 10.sup.-11 cm/watt in fused silica. For instance, in order to achieve a gain of 0.3 dB/km for 1.56 .mu.m signal radiation in a single mode silica-based fiber of core area of 25 (.mu. m).sup.2, pump power of the order of 100 mW is required, if the pump wavelength is about 1.46 .mu.m.
It is also known that Stimulated Brillouin Scattering (SBS) can take place in optical fibers, and that such scattering can have a deleterious effect on systems performance, due principally to the fact that SBS can cause severe fluctuations in the pump intensity, which cause corresponding fluctuations in the Raman gain and to the fact that SBS can result in pump depletion. See, for instance, R.H. Stolen, op.cit. SBS can have a peak gain that is several hundred times that for SRS, per unit frequency of pump radiation, but SBS linewidths are typically very narrow, e.g., of the order of 20 MHz.
G. A. Koepf et al, Electronics Letters, Vol. 18(22), 1982, pp. 942-943, report on Raman amplification at 1.118 .mu.m in single mode fiber and its limitation by SBS. They observed a deleterious effect of SBS on the Raman gain and suggest, inter alia, that an increase in the spectral width of the pump laser by modulation to values larger than the Brillouin linewidth would cause a decrease of the SBS gain and could be used for suppression of Brillouin scattering. See also E. P. Ippen and R. H. Stolen, Applied Physics Letters, Vol. 21(11), pp. 539-541 (1972), which reports on the observation of SBS in optical fiber.
D. Cotter, Electronics Letters, Vol. 18(15), 1982, pp. 638-640, discloses a technique for suppression of SBS during transmission of high power narrowband laser light in monomode fibers. The technique involves imposition of phase modulation on the optical field launched into the fiber so as to reduce the SBS gain. This is achieved, for instance, by placing between the laser and the fiber a periodically driven optical phase modulator, or by using a mode-beating effect produced when the radiation field comprises two discrete but closely spaced optical frequencies. This, it is suggested, could be achieved by using two single-frequency lasers operating at slightly different wavelengths, or perhaps more easily by using a single laser which is arranged to operate in two longitudinal modes. This principle was applied by J. Hegarty et al, Electronics Letters, Vol. 21(7) 1985,pp. 290-292, who used a laser operating in two modes separated by 2 GHz.
Although SRS can be used to amplify "linear" pulses, i.e., pulses in which no particular relationship between pulse peak power and pulse peak width is required, amplification by SRS can be advantageously used in soliton communications systems. A. Hasegawa et al have shown (Applied Physics Letters, Vol. 23(3), pp. 142-144 (1973)) that under certain conditions shapemaintaining pulses can exist in single mode optical fiber. Such pulses are termed solitons, and, in silica-based fiber, typically have center wavelengths in the range 1.45-1.60 .mu.m. The existance of solitons has been experimentally demonstrated (L. F. Mollenauer et al, Physical Review Letters, Vol. 45(13), pp. 1095-1098 (1980)), and their utility for high capacity communications systems has been disclosed (U.S. Pat. No. 4,406,516, issued Sept. 27, 1983 to A. Hasegawa, co-assigned with this). Furthermore, it has been found that solitons can be amplified nonelectronically without loss of soliton character (see A. Hasegawa, Optics Letters, Vol. 8, pp. 650-652 (1983), incorporated herein by reference). Co-assigned U.S. Pat. No. 4,558,921 discloses a soliton optical communications system comprising nonelectronic means for increasing the pulse height and decreasing the pulse width of soliton pulses. See also A. Hasegawa, Applied Optics, Vol. 23(19), pp. 3302-3309 (1984) incorporated herein by reference. This coupling between pulse height and pulse width is an attribute of solitons, and its existence has been experimentally verified in single mode fiber, with loss compensated by Raman gain. (L. F. Mollenauer et al, Optics Letters, Vol. 10, pp. 229-231 (1985).)
Since Raman amplification of signal pulses in fiber communications systems, especially in soliton systems, potentially has substantial advantages over pulse regeneration as currently practiced, a Raman amplification scheme that, among other advantages, avoids the introduction of significant amounts of SBS-caused pump noise yet is easily and inexpensively implemented would be of considerable interest. Co-assigned U.S. Pat. No. 4,699,452 entitled "Optical Communication System Comprising Raman Amplification Means" discloses such a system.
For all optical, soliton based, long distance transmission systems such as the one disclosed in the '452 patent, it has been established that Raman gain depends upon the relative polarization of the pump signal and the transmitted signals in the fiber. Unfortunately, it has been discovered by me experimentally that the dispersion associated with birefringence in ordinary optical fibers such as those suggested in the '452 patent is not of sufficient magnitude to cause a thorough averaging of the relative polarization. As a result, system gain tends to fluctuate nondeterministically with temperature, fiber twist, and other disturbances. A related and, perhaps, even more significant result of this problematic discovery is that the dispersion of birefringence, also known as polarization dispersion, limits the ultimate transmission speed in the system if left uncorrected.